1. Field of the Invention
The field of the invention is directed to the melting of steel and, in particular, to the foaming of slag to improve the steel making process.
2. Description of the Prior Art
Historically in open hearth steel making processes, the principal purpose of slag that forms on the surface of the molten steel is to remove undesirable impurities during the refining period after all of the components of the "charge" of ore, scrap or other steel sources had become molten. This so-called "basic practice" occurs by the introduction of burned lime (CaO) as one of the components of the furnace charge. Steel scrap, coke, some ores and hot metal from blast furnaces are other slag creation sources. A typical furnace is usually lined with a refractory made from the oxides of magnesium and chrome which have higher melting points than the charge materials. The slag and furnace lining, both being basic pH in nature, do not attack one another chemically. The molten steel is more or less neutral and only erodes the furance refractory from the high temperature and the movement of the molten steel during the furnace tap or emptying step.
With the advent of the electric arc furnace in steel making, furnace slag took on additional functions. The heat introduced inside the furnace is from the arc of typically three carbon electrodes creating temperatures estimated to be in excess of 6,000.degree. Fahrenheit. This is much higher than the melting point of the furnace refractory, therefore after the scrap or raw steel charge is completely melted the sidewalls of the furnace are subject to exposure to this intense heat except for the portion of the arc that can be submerged below the slag cover that is created on top of the molten metal. The practice heretofore was to use an amount of lime (CaO) for sulphur and phosphorus removal and also to cover the arc as much as possible. Too much lime would slow the overall melt process time, since lime has a high melting point and also the resulting slag would be viscous in nature and would not remove the undesirable impurities effectively. This added function of the slag was to shield the furnace sidewalls from the effects of the electrode arc and insulate the molten bath from losing its temperature into the atmosphere via the furnace dust control evacuation system positioned above the furnace. The furnace operators would try to compromise their methods during the refining period by retarding the power input through the electrodes and shortening the arcs to keep them below the slag level. This would save on refractory wear but would lengthen refining times and consume more electrodes and kilowatts of electricity. Thus, melt shops had to determine practices which would economically obtain profitable end results by weighing furnace availability and refractory costs against electrode and kilowatt consumption as well as the time element to complete a steel melting process.
Others have considered the use of foaming agents to be added to the slag and a variety of products have entered the marketplace such as coke or some type of carbon product which is introduced into the furnace as the refining period was begun. Such prior foaming agents would increase the depth of the furnace slag as much as three-fold and hence, the arc would be completely submerged. The power factor was increased to a range never attained before with a longer arc and no damage occured to the furnace refractory as long as the foaming slag was maintained. The problem with using materials of the prior art is that the duration of the foaming action is relatively short and subsequent additions of foaming agent have to be made. This is accomplished by either shoveling carbon or coke materials into the door of the furnace or pneumatically injecting the material through a hole in the furnace sidewall above the slag line. This present method is labor intensive and costly of carbonaceous materials.